In so-called standby uninterruptible power systems, it is essential that deviations of the AC line voltage outside the allowed band be detected quickly, ideally instantaneously, in order to transfer the protected load to an alternative power source in the shortest time possible.
Based on the above performance requirements, we can define the ideal AC power detector as follows: the detector should be responsive to the instantaneous voltage value rather than its average or RMS in order to allow fast response; the detector should be capable of detecting both positive and negative deviations of the monitored AC source; and the detector should be able to react very quickly for large errors and slowly for small errors. Such characteristics will allow the detector to ignore, if desired, irrelevant errors such as harmonic distortions and short term, small magnitude, voltage deviations.
In the prior systems, the requirements set forth above are approximately satisfied by circuits based on the assumption that the monitored voltage is sinusoidal. Consequently the monitored waveform value is sampled at one, two, or more points and a decision is made that the monitored voltage is within tolerance when the samples are within a certain range.
There are numerous variations of detector circuits employing this principle. A typical one is described by H. Yamashita in his paper "Uninterruptible Power Systems in NTT" (Proceedings in INTELEC 90, pp. 414-415, IEEE publication 3CH2928-0/90/000/0412). The waveforms relevant to the detector proposed by Yamashita appear in FIG. 1. As stated by Yamashita, the AC voltage 10 is rectified. The rectified voltage 20 is compared with a reference level 22, and a train Tv of pulses 24 is obtained. The width of pulse 24 is compared with the width of reference pulse 26 of train Tr. When commercial power failures occur, the width of pulse 24 Tv will be changed, and the detection circuit will then detect the voltage failure.
Inspection of this type of circuit reveals that it has some serious deficiencies. Referring to FIG. 2, it can be seen that a severely clipped waveform 25 will not be treated as a failure by the detector, while a small distortion 30 around the sampling point that may be inconsequential, particularly if it is a one time phenomenon, will cause an immediate failure detection.
In contrast to the foregoing, the voltage detector of this invention eliminates these deficiencies and others existing in the prior art. In addition it provides the capability of sensing phase and frequency deviations of the monitored voltage source.